I believe the Corporate GM vacuum shift rears were used until about 1972. I didn't have any problem getting a new diaphram for my '56 vacuum actuator few years back...parts man said it fit lots of years.
The push/pull cable will have a 4-port switch on the bottom end. One port is the supply, one for Hi range, one for Low range and one small port for the speedometer 2-speed adapter (mounted to the back of the speedometer).
Like Bill said, try your local big truck junk yard.
Mike B
